The present invention relates to an apparatus for detecting the light focus position, more particularly, to an apparatus for detecting the light focus position on an optical scanner in which the light beam as deflected by a rotating deflector is focused on the surface to be scanned.
Laser light as modulated by image information is deflected by a rotating polygonal mirror or other deflecting means and the deflected laser light is focused and scanned over a photoreceptor to form a latent electrostatic image on the latter. This operating principle is used in a common type of optical scanners such as a laser printer.
Unless the deflected laser light is in sharp focus on the photoreceptor, the optical scanner is unable to produce quality printing and, hence, focusing is an important consideration. Various proposals have heretofore been made in connection with the improvement in such focusing.
One of the proposals made to date is described in Unexamined Published Japanese Patent Application (kokai) No. Hei. 1-237614 and it relates to a technique in which two light detectors each having a slit are provided on an extension line of the surface to be scanned of the photoreceptor, one being in an upstream position and the other being downstream, and in which the laser beam is focused on said surface in response to a signal as detected with said light detectors.
Another proposal is described in Unexamined Published Japanese Patent Application (kokai) No. Hei 4-155304 and it relates to a technique in which two knife edges are provided on an extension line of the surface to be scanned of the photoreceptor, one being in an upstream position and the other being downstream, and in which the laser beam as cut with said knife edges is admitted by a light receiving element, with an output detection signal from said receiving element being differentiated to detect an offset in the focus position and an adjustment being subsequently made in such a way that the light beam will be properly focused on said surface to be scanned.
The first of the two conventional techniques just mentioned above requires two light detectors and it is necessary to provide slits of a smaller width than the incident light beam but this increases the chance of erroneous operation due to such reasons as the deposition of dust particles. As a further problem, the two light detectors must be provided in positions that are offset from the photodetector but which lie on the extension line of its surface to be scanned; to meet this requirement, a large space is necessary.
The second conventional technique uses only one light receiving element; on the other hand, the direction of offset in the focus position is detected by differentiated components of the detection signal and, as a result, any variations or the like in the characteristics of light detectors will make it impossible to adjust the light focus position with high accuracy.